Analysis of semiconductor surfaces by secondary ion mass spectrometry and methods

ABSTRACT

A method for a mass spectrometric determination of contaminant components of a thin oxide surface layer of a semiconductor wafer use a movable mechanical stage to scan and raster a large area of the wafer in a continuous scanning motion. The mass of analyte is greatly increased, resulting in improved sensitivity to trace components in the surface layer by a factor of 10-100 or more. A light beam interferometer is used to determine non-planarity from e.g., warping of the wafer and provide a correction by maintaining a constant separation between the wafer and the extraction plate or adjusting the electrical bias of the wafer relative to the extraction bias.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/358,939,filed Feb. 4, 2003, pending, which is a continuation of application Ser.No. 09/795,999, filed Feb. 28, 2001, now U.S. Pat. No. 6,528,786, issuedMar. 4, 2003, which is a continuation of application Ser. No.09/309,208, filed May 10, 1999, now U.S. Pat. No. 6,232,600, issued May15, 2001, which is a continuation of application Ser. No. 09/035,197,filed Mar. 5, 1998, now U.S. Pat. No. 5,920,068, issued Jul. 6, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the bulk measurement of tracecontaminants in the surface layers of semiconductor wafers and dies, aswell as material composition as a function of depth. More particularly,the invention pertains to improvements in methods and apparatus for massspectrographic analysis of wafer and semiconductor die surface layers.

2. State of the Art

Secondary ion mass spectrometry (SIMS) is known as a method fordetermining particular constituents of a semiconductor material andproviding a quantitative measurement of each.

Generally, this method involves bombarding a sample with “primary” ions,e.g. oxygen ions, measuring the intensities of secondary ions emitted orsputtered from the sample, and calculating the quantity of eachconductive impurity based on the secondary emission as compared to theemission of standard materials. The sputtering and analysis is typicallyconducted in an ultra-high-vacuum environment.

SIMS may be used to achieve parts-per-billion (ppb) detection limits forbulk analysis and for determining material composition as a function ofdepth, provided the sample size is sufficiently large. The extremesensitivity of SIMS results from its ability to “consume” large amountsof sample material, and thus process a large number of atoms to detect.However, because of the high rate of material consumption from a verysmall sample, dynamic SIMS is generally not appropriate for analysis ofa very thin oxide surface layer of a semiconductor die and plurality ofsemiconductor dice in wafer form. Typical semiconductor contaminants mayinclude lithium, boron, sodium, potassium, iron, sulfur, and carbon, allof which are found in the oxide layer on the semiconductor die surface.For the case of surface contaminants on silicon, the oxide layer isgenerally not more than about 15 Å thick. However, several minutes arerequired to obtain a sufficient number of data points at the requiredanalyte masses, so the method is not useful for this application as theoxide layer will be quickly consumed.

It is desirable to be able to detect the concentrations of boron,lithium and sodium to less than about 1×10⁶ atoms per square centimeterof semiconductor die surface area. These detection limits areconsiderably lower than currently obtainable.

U.S. Pat. No. 4,874,946 of Kazmerski discloses a method and apparatusfor mapping the chemical composition of a solid device, using arasterable SIMS mass analyzer.

U.S. Pat. No. 4,611,120 of Bancroft et al. discloses a method forsuppressing molecular ions in the secondary ion mass spectra of acommercial SIMS instrument.

U.S. Pat. No. 5,521,377 of Kataoka et al. discloses a method foranalysis of a solid in a planar or depth-wise direction using sputteringwith two ionizing beams and detecting a two-atom composite ion.

U.S. Pat. No. 5,502,305 of Kataoka and U.S. Pat. No. 5,442,174 ofKataoka et al. disclose methods for analysis of a solid in a planar ordepth-wise direction using sputtering with an ionizing beam anddetecting a three-atom composite ion.

U.S. Pat. No. 5,332,879 of Radhakrishnan et al. discloses the use of apulsed laser beam to remove contaminant metals from the surface of apolyimide layer. The disclosure indicated high surface metal removalwith “minimal” removal of the polyimide, i.e., 250-500 Å per pulse. Suchablation rates are far greater than useful in the analysis of surfacecontaminants in semiconductor devices, where the surface oxide layer istypically only about 15 Å in depth.

Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has also beenfound useful for bulk analysis of materials, provided the sample size issufficiently large. The TOF-SIMS instrument directly measures the speedsof secondary ions by measuring the time taken to travel a givendistance. Knowing the ion's energy, which is defined by thespectrometer's acceleration voltage, its mass can then be calculated.Typically, the time intervals are defined as the difference in timebetween pulsing the ion gun and the ion arrival at the detector. Themass range is then calibrated using at least three known mass peaks.

TOF-SIMS instruments have been found to provide some of the lowestdetection limits in surface analysis, typically even lower than totalreflection X-ray fluorescence (TXRF) with vapor phase decomposition(VPD). For the TOF-SIMS instrument, some representative detection limitsare <1×10⁸ atoms/cm² for lithium, boron and sodium, and <1×10⁹ atoms/cm²for iron.

The TXRF instrument, on the other hand, is incapable of detectingelements lighter than sulfur, so critical elements such as sodium,carbon, lithium and boron cannot be detected.

Thus, the TOF-SIMS method would appear to be potentially useful forsurface analysis, but instrumental constraints limit the sampling areato about 100×100 μm, and sampling of a relatively shallow oxide layerover the 100×100 μm area does not produce sufficient sample material forachieving the desired detection limits.

For TOF-SIMS, the detection limits are determined by the transmissionand exceptance of the mass spectrometer, the sputter and ionizationyield of the analyte, and the amount of material consumed during theanalysis. These parameters may be categorized as the useful yield of themass spectrometer and volume of analyte. Sampling of the maximum rasterarea of 100×100 μm to a depth of about 13 Å will produce about 3×10¹¹particles. This is equivalent to between 0.3 to 30 (thirty) counts of ameasured component at the 1 ppm level depending upon the ionizationyields. It is critical to semiconductor device manufacture that bulkconcentrations of some contaminants as low as 0.01 ppm and even 1 ppb beaccurately detectable. Thus, current detection limits for certaincontaminants must be reduced by a factor on the order of about 100 ormore.

U.S. Pat. No. 5,087,815 of Schultz et al. discloses a method andapparatus for a TOF-SIMS isotopic ratio determination of elements on asurface.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for substantially increasing thesensitivity of a mass spectrographic analytical method for determiningcontaminant levels in the surface oxide layer of a semiconductor die orsemiconductor dice in wafer form.

The present invention further provides a method for increasing thesensitivity of surface contaminant analysis of an oxide layer of a waferby secondary ion mass spectroscopy (SIMS).

The present invention additionally provides a method for increasing thesensitivity of surface contaminant analysis by time-of-flight secondaryion mass spectroscopy (TOF-SIMS).

Related to the present invention is providing a method for determiningthe bulk concentration of contaminants in a surface oxide layer of asemiconductor material by SIMS or TOF-SIMS, wherein the surface areawhich may be sampled may be much greater than the electrostatic rasterlimits of the sputtering primary beam, and/or acceptance area limits ofthe spectrometer.

The present invention further includes apparatus for achieving thedesired rastered sputtering and analysis of an enlarged area of asemiconductor wafer.

The present invention includes a method and means which enablesputtering to a uniform sampling depth and maintaining mass resolutionirrespective of warpage or other non-planarity of a wafer. Thus,contaminant analysis of semiconductor wafers and semiconductor dice bySIMS or TOF-SIMS may be limited to the surface oxide layer.Additionally, advantages and novel features of this invention are setforth in part in the description infra. These advantages and featureswill become apparent to those skilled in the art upon examination of thefollowing specification and drawings, or may be learned by practice ofthe invention. The various combinations of apparatus and/or methodswhich comprise the invention are pointed out in the appended claims.

In accordance with this disclosure, a first aspect of the inventioncomprises the steps of:

(a) scan-sputtering a large area of a semiconductor wafer surface whilecontinuously moving, i.e. scanning the wafer with a supportingmechanical stage in a first direction, and mechanically rastering thewafer in a second direction, whereby the scanning speed is controlled tolimit the sputtering to a sampling depth not generally exceeding thedepth Q of the surface oxide layer, and the rate of sputtered secondaryionic emission directed to the SIMS detector simultaneously satisfiesthe SIMS consumption rate; the total sputtered area is generally atleast about 10⁴ μm² for a surface oxide layer sputtered to a depth ofabout 15 Å;

(b) directing a stream of secondary ions produced by the primaryionizing beam into a SIMS for mass spectrographic analysis at a ratesatisfying the SIMS sample consumption rate for a time period sufficientfor high analytical sensitivity; and

(c) computing the total mass of each of the selected detected ions.

In a second aspect of the invention, a combination of mechanicalscanning/rastering and primary beam electrostatic rastering is used tomove the sputtering and sampling operations over the wafer surface at aspeed responsive to a controlled sampling depth and sample consumptionover a large area.

In a third aspect of the invention, mechanical scanning and rasteringare used to sample a large area of a semiconductor material with acontinuous sputtering and analysis by a time-of-flight secondary ionmass spectrometer (TOF-SIMS).

In a fourth aspect of the invention, a combination of mechanicalrastering and primary beam rastering, e.g. electrostatic rastering isapplied to a time-of-flight secondary ion mass spectrometer (TOF-SIMS)whereby a large area of controlled limited depth may be sputtered at aspeed which simultaneously (a) limits total sputtering depth generallyto the surface oxide layer and (b) provides a large quantity ofsecondary ions for high resolution of measured atoms.

In one preferred method, an area, typically limited by the electrostaticrastering capability of the TOF-SIMS instrument to about 100 μm², isrepetitively sputtered and analyzed. The primary beam is then moved to anew area by mechanical stage rastering, and the new area is repetitivelysputtered and analyzed using electrostatic rastering. The process isrepeated until sufficient sample material is consumed for each “slice”of the surface oxide layer to provide the desired detection limits. Thetotal area sampled is limited only by the total area of the wafer or diebeing analyzed, and the time available for analysis.

Thus, the detection limits are reduced to low levels. For example, thedetection limits of lithium, boron, and sodium may be extended to lessthan about 1×10⁶ atoms/cm².

The detection limits of trace iron may be at somewhat higherconcentrations because of the peak shape of the neighboring SiO₂.

In a variant of the above method, mechanical rastering is performed suchthat a thin slice of an enlarged area, e.g.

400-1600 μm is sputtered and analyzed before the next slice is sputteredand analyzed.

In each of the embodiments described and illustrated of the presentinvention, it should be kept in mind that removal of the surface layerby sputtering is conducted in a continuous scanning motion rather thanby stepping along a series of stationary raster points. The sputterdepth is controlled by the scanning speed, and the sputter rate (massper unit time) is controlled by the ion gun characteristics.

Because of the much greater area which is rastered, sputtered andanalyzed, the analysis is more sensitive to non-planarity, e.g. warpage,of the wafer. Non-uniform extraction fields and some loss of massresolution inaccuracy in analysis may result. This would have adverseeffects on detection limits of Fe since the neighboring peak of Si₂ isat the same nominal mass as Fe and will overlap Fe without sufficientmass resolution.

Thus, in another aspect of the present invention, a method and apparatusare provided for counteracting the effects of wafer warpage(non-planarity) upon analytical results. In accordance with theinvention, a non-invasive laser interferometer is incorporated into theSIMS or TOF-SIMS spectrometer to measure non-planarity as a function ofwafer location, and permit correction of the stage elevation or thesample electrical potential relative to the extraction potential. Thesample elevation measurements may be made prior to the massspectrometric analysis and then used to control e.g. the stage elevationduring rastering/sputtering, or the sample elevation measurements may bemade during the rastering/sputtering operation for continuous in-situcorrection. Because of the required physical separation, e.g. about 1-3cm., of the sputter beam and the interferometer laser beam in the lattercase, the sample surface elevation at the sputter location may becalculated and correction made therefor by a computer program of theprocess controller, based on distant measurements and assuming aparticular warpage shape. Alternatively, the wafer may be first scannedby the interferometer; the data may be stored and used to providecorrection during the sputtering/analyzing operation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention is illustrated in the following figures, wherein theelements are not necessarily shown to scale.

FIG. 1 is a front diagrammatic view of an exemplary secondary ion massspectrometer (SIMS) apparatus of the invention for high resolutiondetection of component concentrations in a thin surface oxide layer of asemiconductor die or wafer;

FIG. 2 is a perspective view of a portion of a semiconductor waferillustrating a method of the invention for determining the componentconcentrations in the surface oxide layer thereof;

FIG. 3 is a lateral diagrammatic view of a laser interferometer usefulin the invention;

FIG. 4 is a plan view of a portion of a semiconductor wafer,illustrating an exemplary scanning/rastering pattern of the invention;

FIG. 5 is a side diagrammatic view of an alternative grazing angleinterferometer used in the present invention;

FIG. 6 is a graphical representation of the grazing property of thegrazing angle interferometer used in the present invention;

FIG. 7 is a graphical representation of the increase in reflection ofthe beam as the grazing incidence angle is increased; and

FIG. 8 is a table of wafer surfaces capable of measurement by a grazingincidence interferometer.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings of FIGS. 1-4, and particularly to drawingFIG. 1, a secondary ion mass spectrometer (SIMS) analysis system 10 ofthe present invention is illustrated. The SIMS analysis system 10includes a primary sputtering ion generator or ion gun 12, a samplestage 14 having a stage controller 16, and a secondary ion analyzer 18.A primary ion beam 20 is directed from the ion generator 12 onto thesurface 22 of a semiconductor die or wafer 24 mounted on the stage 14,and a stream of secondary ions, represented by line 26, is sputteredfrom the die 24 by the primary ion beam 20, extracted by the extractionfield between the wafer 24 and the extraction plate 151 and directedthrough an electric field 28 and a magnetic field 32 for separation ofions, i.e. resolution of the ion stream. The stream 26A of partiallyresolved secondary ions is then directed through a collector slit 36 anddeflection electrode 38 to a spectrometric detector 30. The detector 30is conformed to detect the resolved secondary ionic emission stream 26Aand relay measured values via signal conductor 34 to acontroller/analyzer/recorder 40. Other time-dependent inputs to thecontroller/analyzer/recorder 40 include an indication of the samplinglocation from the stage controller 16 via conductor 42. Thecontroller/analyzer/recorder 40 controls the ion gun 12 via signalconduit 43.

The controller/analyzer/recorder 40 receives the measured values ofcurrent measurements from signal conductor 34, determines the particularsecondary ions thereof, and sums the counts to provide a “bulk”concentration of each ion, and/or a concentration of each as a functionof X-Y location and/or an interval of depth Z in the wafer or die 24. Inaccordance with the method of the invention as generally practiced, thewafer or die 24 has a surface oxide layer 44 in which are thecontaminant atoms/molecules of interest, and the depth of sputteringdoes not generally exceed the depth 46 (FIG. 2) of the surface oxidelayer.

As used herein, the term “rastering” refers to the movement of a primaryion beam, laser light beam, or other apparatus or process in a line overthe surface of a wafer or other planar material, and lateral movement,in turn, to each of a plurality of subsequent lines, whereby asignificant area of the surface is subjected to the apparatus or processof the present invention.

The functions of the controller/analyzer/recorder 40 need not beconfined to a single instrument, but may be distributed, for example, inthe ion generator 12, stage controller 16, spectrometric detector 30,and/or controller/analyzer/recorder 40.

The primary ion beam 20 may be a stream of e.g. O₂ ⁺ ions, Cs⁺ ions, orother ions, as known in the art. As also known in the art, the primaryion beam 20 may be laterally focusable by an electrostatic lens 48 toincrementally raster the ionization of the wafer surface 22. The maximumarea of the wafer surface 22 which is electrostatically rasterable underthe constraints of currently available SIMS equipment has dimensions ofabout 100×100 μm. In some configurations of the present invention, theprimary ion beam is not rastered.

In one aspect of the invention, a movable stage 14 with a stagecontroller 16 is provided to support the wafer or die 24 and to provideboth a controllable scanning motion in a first direction and a rasteringmotion of the wafer or die in a second direction. The stage 14 ispreferably movable in the lateral X and Y axes at a controllablevariable speed, such as between about 200 and 1000 μm/sec., and ismovable along the Z axis. The surface area which may be sampled is thusnot limited by the electrostatic beam-raster.

In another aspect of the present invention, a movable stage 14 with astage controller 16 is provided to support the die or wafer 24 and toprovide a controllable scanning and raster pattern in combination withthe electrostatic rastering of the electrostatic lens 48, greatlyincreasing the total rasterable area thereby.

In drawing FIGS. 1 and 2, stage 14 is an automated sample stage whichpermits e.g., scanning and rastering of wafers of any size typicallyused in the industry, such as 8 inches (20.3 cm.) in diameter. The wafersurface 22 is sputtered by primary ion beam 20 while scanning at a ratewhereby the total sputtering depth does not generally exceed the depth46 of the surface oxide layer 44 on the wafer surface. Thus, a bulkanalysis at lower concentration limits may be obtained for the surfaceoxide layer 44.

An example of a raster pattern is shown in drawing FIG. 2 whereinstraight-line scanning along line or path 94 is followed by rastering tothe adjacent line 96 and reversing the scanning direction. This isrepeated across the surface 22 until the entire area to be sampled issputtered to the control depth 45 and analyzed.

If desired, the scanning may be at a speed and/or primary beam strengthwhereby only a fraction of the surface oxide layer 44 is removed andanalyzed per pass. Thus, a bulk analysis of the trace components may beobtained which will have limits equal to or lower than conventional bulkanalyses.

For determination of composition as a depth function, the wafer surface22 is repeatedly and progressively scanned in multiple passes, and thesecondary ion stream from each pass is analyzed and correlated as afunction of X-Y position and depth (Z position).

Electrostatic beam rastering may be added to a mechanicalscanning/rastering. In this configuration, a beam rasterable area isscan-sputtered (as opposed to point-sputtered). Control of the rasteringand scanning motions is combined whereby a much larger surface area maybe scanned and rastered.

As shown in drawing FIG. 4, a surface 22 of a wafer 24 issputter-sampled and analyzed by an apparatus of the present invention.Sector A comprises the maximum area electrostatically rasterable by theion gun 12 and is shown as a square pattern having dimensions 100 and102 of e.g. 100 μm×100 μm. Sector A is bounded by lines 104, 106, 108and 110. Adjacent sectors B, C, and D are shown in part, each havingdimensions equal to sector A.

The exemplary raster pattern of the ion beam starts at point 120adjacent corner 112, and progresses along line or path 114 to point 122adjacent corner 116. At that location, the beam is rastered along line118 to point 124, and sputtering continues along line 126 to point 128.The ion beam 20 is continuously moved at a speed and primary beamstrength sufficient to remove and ionize the surface 22 to the desireddepth. The secondary ion stream 26 is directed into the secondary ionanalyzer 18. The scanning and rastering of the primary ion beam 20 iscontinued over the entire sector A to point 130 adjacent corner 132.

The wafer 24 is then mechanically rastered by the movable sample stage14 in direction 134 so that sector B occupies the area previouslyoccupied by sector A. At the same time, the primary ion beam 20 is movedin directions 134 and 136 to focus on point 138 adjacent corner 140, andbegin a scanning, sampling and analysis of sector B. These operationsmay be continued for any number of sectors, limited only by the size ofthe wafer 24 and the time available for the analysis.

Thus, for example, a first analysis area of 200×200 μm may be scanned ata speed to complete sputtering in 0.5 second. The analysis depth will beabout 13 Å which is close to the depth of the surface layer. A 300 nÅprimary probe beam will consume the sputtered mass (about 3×10¹¹particles) from this sputtered area (4×10⁴ μm²) in about 0.5 seconds,and yield accurate bulf analyses of contaminants at low concentrationlevels. Following sputtering of this 200×200 μm area, the wafer ismoved, i.e. rastered, by the mechanical stage to a second area,typically of equal size and adjacent the first area, which is thenscanned. A plurality of e.g. 100 or more area may be thus sampled andanalyzed, depending upon the wafer size, greatly increasing thesensitivity to contaminant levels. As described above, the primary ionbeam electrostatic raster is not used; all movement is controlled by themechanical stage.

Alternatively, the electrostatic rastering and mechanical rasteringdevices may be combined to provide a linear scanning motion forcontinuous sputtering and a raster step motion for moving the primaryion beam 20 to a new line of sputtering. A single pass, for bulkanalysis, or multiple passes for depth analysis, may be performed.

Other raster patterns may be used, and the number of sectors chosen toraster by this method may be any convenient number which provides thedesired analytical accuracy.

Consumption of the substrate underlying the surface oxide layer 44 isavoided, increasing the concentrations of contaminant ions to bemeasured and reducing the analytical limits by a factor of about 10 to100 or even more. Thus, surface analysis may be achieved with the samesensitivity as a typical SIMS bulk analysis.

For surface analysis of large areas such as on a wafer of 8 inches orgreater diameter, any surface non-planarity resulting from e.g. warping,will affect the extraction voltage, and hinder mass resolution and thusabundance sensitivity. Such warpage on an 8 inch wafer is typicallyspecified to be less than about 40 μm across the wafer. Thus, theallowable non-yield planarity would yield up to a 2% change inextraction field for extraction gate. Unless corrected, the resultingerror may cause an unacceptable loss in mass resolution and abundancesensitivity.

Thus, in a further feature of the invention, an interferometry apparatusand method are combined with compensation apparatus and methods forcounteracting sample non-planarity.

Drawing FIGS. 1-3 illustrate this feature as configured in accordancewith the known Michelson type of interferometer 50. A light beam 54generated by a collimated light source 52 such as a laser is directed ata beamsplitting surface 56 of a beamsplitter 60. A first split beam 62is reflected from the beamsplitting surface 56 to the wafer surface 22and is reflected back to the beamsplitter 60, passing through it to adetector 70 as beam 64. A second split beam 66 passes through thebeamsplitter 60 and anti-reflection surface 58 to a reflecting surface68 of an axially movable mirror 72, from which the beam 66 is reflectedback to the beamsplitter 60. The reflected beam 66 is further reflectedfrom the beamsplitting surface 56 and passes to the detector 70 as partof beam 64. Depending upon the difference in distance traveled by thefirst split beam 62 and by the second split beam 66, the two beamportions received as beam 64 by the detector 70 will interfereconstructively or destructively. Compensating plate 74 compensates forthe thickness of the beamsplitter 60, and may be unnecessary when thegenerated light beam 54 is monochromatic or quasi-monochromatic. In apreferred embodiment, the generated light beam 54 is a coherent laserbeam.

The function of the beam splitter is to superimpose the mirror imagesonto each other and determine the phase difference and hence thedifference in distance traveled by the first and second split beams 62,66. The difference in traveled distance is twice the control distance76. Sample distance 78 equals the total of control distance 76 andmirror distance 80. The detector 70 measures the change in amplitude ofthe beam 64 from which the difference in phase between beams 62 and 64may be calculated and counts interference fringes. A warped wafer 24will change the sample distance 78 and this change in distance relativeto the mirror distance 80 (or the control distance 76) will be detectedand quantified by the detector 70. Mirror controller 90 provides preciseaxial movement 88 of mirror 72, controlling distance 76 for determiningthe phase difference and may be used for phase modulation. Signal lines82, 84, and 86 interconnect the light source 52, detector 70, mirrorcontroller 90 and the controller/recorder/analyzer 40. Conductor 42provides for vertical movement of stage 14, by which the stagecontroller 16 raises or lowers the wafer/die 24 to compensate formeasured warpage or other source of non-planarity. Alternatively, the DCbias of the wafer/die 24 relative to the extraction plate 151 may bevaried, i.e. raised or lowered to maintain a constant extraction field.

Movement of the stage 14 by the stage controller 16 may be continuouslycontrolled in each of the X, Y and Z axes, so that the primary ion beam20 may be scanned and rastered over the desired X-Y sample area 98 andthe ion gun may also be maintained at a relatively constant distancefrom the wafer surface 22. Thus, the surface 22 will be sputtered to arelatively constant depth 45, constant extraction field, and sputteringwill be substantially confined to the oxide layer 44, which is a desiredresult of this method.

While a Michelson type of laser interferometer is illustrated in drawingFIGS. 1 and 3, the interferometer may alternatively be a “grazingincidence” interferometer of the Twyman-Green type or may be of anotherconfiguration, e.g. Mach-Zehnder, Shearing, or Fabry-Perotinterferometer. Also, by further using phase modulation on theabove-referenced interferometers, the absolute position of the wafer canbe evaluated using well known Fourier transform calculations. Thevarious types of interferometer generally differ in the beam geometriesand in analytical treatment of the response. Incorporation of aparticular type of interferometer into a SIMS or TOF-SIMS apparatusshould be made with measurement accuracy and ease of operation in mind.

Placement of an interferometer in a mass spectrometer will require thatthe first split beam 62 be separated from the primary ion beam 20. Thisseparation distance 92 may be e.g about 0.5-2 cm. This problem may beovercome by initially scanning the wafer surface 22 with theinterferometer 50, storing the planarity results, and providingcontinuous or semi-continuous correction during the sputteringoperation. Alternatively, in situ remote interferometric measurementsmay be taken during the sputtering operations and corrections made basedon calculated deviations from planarity.

Referring to drawing FIGS. 5 through 8, another type of interferometer,a “grazing incidence” interferometer 100 is illustrated for use in thepresent invention. By using a grazing incidence interferometer 100 andby using “phase modulation” techniques the basic data generated fromscanning the surface of a wafer is readily transferred to a computer foranalysis through the use of well known Fourier series phase calculationtechniques to determine the degree and direction the wafer is warped.

As illustrated in drawing FIG. 5, the grazing incidence interferometer200 typically includes a laser light source 202, objective 204,coherence limiter 206, GALVO 208, fold mirror 210, fold mirror 212,collimator lens 214, fold mirror 216, prism 218, fresnel lens 220, foldmirror 222, fold mirror 224, video (TV) camera 226, video output 228,and computer interface 230. Laser beam 150 is illustrated emanating fromlaser 202, dividing into beams 150′, which are received by video camera226 after being reflected from the wafer 24 supported on a suitablemember (not shown).

Referring to drawing FIG. 6, graphically represented, a laser beam 150is illustrated grazing off the surface of a wafer 24 at a relativelylarge angle to the surface normal through prism 218. Using this methodof “grazing incidence” for the laser beam off the surface of the waferand by using “phase modulation” techniques the basic data is readilytransferred from the video camera (not shown) to the computer interfacefor analysis by the computer using Fourier series phase calculationtechniques to determine the surface variations and direction of thesurface variation. By striking a surface at a shallow angle with thelaser beam, objects which are considered quite diffuse becomenear-perfect reflectors of the laser beam.

Referring to drawing FIG. 7, illustrated is the increase in reflectionas the angle of grazing incidence is increased.

Referring to drawing FIG. 8, illustrated in TABLE 1, are wafer surfacesof wafers of varying materials and associated wafer surface conditionsfor which grazing interferometers have been used to measure.

It will be evident to those skilled in the art that various changes andmodifications may be made in the methods and apparatus as disclosedherein without departing from the spirit and scope of the invention asdefined in the following claims.

What is claimed is:
 1. A method for determining contaminant levels in a surface oxide layer of a semiconductor material on a portion of a wafer, the method comprising: scanning the surface oxide layer of the semiconductor material in a first direction; mechanically rastering the surface oxide layer of the semiconductor material in a second direction; and sputtering a portion of the surface oxide layer to a depth not generally exceeding a depth Q at a sputtering rate in mass per unit time controlled by varying a primary ion beam strength and a depth controlled by varying a scanning speed in length per unit time.
 2. A method for determining contaminant levels in a surface layer on a semiconductor material having a surface non planarity on a portion of a wafer, the method comprising: determining a depth Q for the surface layer; scanning the semiconductor material in a first direction; mechanically rastering the semiconductor material in a second direction; sputtering a portion of the surface layer to a depth not generally exceeding depth Q; measuring the surface non planarity of the semiconductor material; and continuously correcting for the surface non planarity of the semiconductor material during the sputtering.
 3. The method of claim 2, wherein measuring the non planarity of the semiconductor material comprises: directing one portion of an interferometer split beam to an X-Y location on the semiconductor material such that the one portion is reflected back to a detector; directing another portion of the interferometer split beam to a mirror at a known distance such that the another portion is reflected back to the detector; and determining a difference in traveled distance by use of phase modulation and Fourier analysis to determine semiconductor material surface offset.
 4. The method of claim 2, wherein continuously correcting for the non planarity comprises moving the semiconductor material along a Z-axis to maintain an approximately constant distance between a primary ion beam of a mass spectrometer and the surface layer being sputtered.
 5. The method of claim 2, wherein continuously correcting for the non planarity comprises changing an electrical potential of the semiconductor material relative to an extraction potential.
 6. A method of providing a uniform extraction field on a surface oxide layer of a semiconductor material on a portion of a wafer, the method comprising: measuring a non planarity of the surface oxide layer of the semiconductor material; correcting for the non planarity of the surface oxide layer of the semiconductor material; and sputtering the surface oxide layer of the semiconductor material to a substantially uniform depth.
 7. The method of claim 6, wherein measuring the non planarity of the surface oxide layer of the semiconductor material and the sputtering the surface oxide layer of the semiconductor material are performed substantially simultaneously.
 8. The method of claim 6, wherein measuring the non planarity of the surface oxide layer of the semiconductor material is performed prior to sputtering the surface oxide layer of the semiconductor material, and such measurements are used in correcting for the non planarity of the surface oxide layer of the semiconductor material.
 9. The method of claim 6, wherein correcting for the non planarity of the surface oxide layer of the semiconductor material is continuous throughout sputtering.
 10. The method of claim 6, wherein corrections while correcting for the non planarity of the surface oxide layer of the semiconductor material are controlled by a computer program and are at least partially based upon measurements made while measuring the non planarity of the surface oxide layer of the semiconductor material.
 11. The method of claim 6, wherein measuring the non planarity of the surface oxide layer of the semiconductor material comprises: directing one portion of an interferometer split beam to an X-Y location on the surface oxide layer of the semiconductor material such that the one portion is reflected back to a detector; directing another portion of the interferometer split beam to a mirror at a known distance such that the another portion is reflected back to the detector; and determining a difference in traveled distance by use of phase modulation and Fourier analysis to determine an offset of the surface oxide layer of the semiconductor material.
 12. The method of claim 6, wherein correcting for the non planarity comprises moving a mechanical stage along a Z-axis to maintain approximately constant distance between a primary ion beam of a mass spectrometer and the surface oxide layer of the semiconductor material being sputtered.
 13. The method of claim 6, wherein correcting for the non planarity comprises changing an electric potential of the semiconductor material relative to an extraction potential of a primary ion beam. 